In recent years, responding to increasing public concern about global environment issues, there has been a growing demand for, for example, curbing use of thick steel sheets which necessitate relatively large CO2 emission during manufacturing of the steel sheets. Furthermore, in the automobile industry, there has been a growing demand for, for example, lighter-weight vehicles which improve a fuel consumption rate while reducing exhaust gas. For these reasons, steel sheets have been made stronger and thinner.
High-strength steel sheets generally have poor blanking workability and toughness. Therefore, it is desired to develop a high-strength thin which can be used for parts molded by press blanking or for parts requiring toughness or, particularly, for parts that are molded by press punching and require toughness at the same time.
For example, JP 2008-261029 A (PTL 1) describes a steel sheet excellent in blanking workability, which is “a high-strength hot rolled steel sheet excellent in blanking workability, comprising, in mass %, C: 0.010% to 0.200%, Si: 0.01% to 1.5%, Mn: 0.25% to 3%, controlling P to 0.05% or less, further comprising at least one of Ti: 0.03% to 0.2%, Nb: 0.01% to 0.2%, V: 0.01% to 0.2%, and Mo: 0.01% to 0.2%, the balance consisting of Fe and inevitable impurities, and a segregation amount of C at large-angle crystal grain boundaries of ferrite being 4 atms/nm2 to 10 atms/nm2”.
Additionally, WO 2013/022043 (PTL 2) describes a steel sheet excellent in toughness, which is a “high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance characterized by comprising, by mass %, C: 0.04% to 0.09%, Si: 0.4% or less, Mn: 1.2% to 2.0%, P: 0.1% or less, S: 0.02% or less, Al: 1.0% or less, Nb: 0.02% to 0.09%, Ti: 0.02% to 0.07%, and N: 0.005% or less, a balance of Fe and unavoidable impurities, where 2.0≤Mn+8[% Ti]+12[% Nb]2.6, and having a metal structure which comprises an area percentage of pearlite of 5% or less, a total area percentage of martensite and retained austenite of 0.5% or less, and a balance of one or both of ferrite and bainite, having an average grain size of ferrite and bainite of 10 μm or less, having an average grain size of alloy carbonitrides with incoherent interfaces which contain Ti and Nb of 20 nm or less, having a yield ratio of 0.85 or more, and having a maximum tensile strength of 600 MPa or more”.